This invention relates to apparatus and a method for rapidly determining the amount of separation between a target surface and a predetermined reference position, and particularly to apparatus for on-line precise monitoring of the thickness of moving material such as sawn lumber, which is subject to thickness variation throughout its lateral cross-section as a result of having been sawn from a waning portion of a log.
In a typical lumber mill on-line thickness monitoring of boards has, until recently, been performed visually by a human operator who views the sawn boards as they are moved along a conveyor toward the trim saws, and who manually actuates a trim saw to cut boards to the maximum standard length throughout which the board is of acceptable thickness over at least the minimum acceptable width. However, as will be readily appreciated, the human eye is not particularly adapted for precise monitoring of board thickness under such circumstances, especially when boards are moving at relatively high speed. A small error in operator judgment consistently applied could result in waste of large quantities of wood. It is therefore highly desirable that an on-line means be provided for automatically and precisely monitoring the thickness of the sawn boards at a point before the trim saws, and using such measurement to determine the length to which the boards are cut.
Previous to the present invention some of the known techniques for on-line monitoring of the thickness of moving material included the use of mechanical fingers and rollers, strain gauges with associated roller sensors, and similar mechanical and electro-mechanical devices using physical contact between the thickness sensing device and the moving material. In addition to their relative imprecision, such prior art devices, because of the frictional wear produced by the continuous contact with the moving material, constantly require readjustment and repair.
Relatively recently, non-contact optical measuring apparatus utilizing coherent light sources (lasers), such as the instruments described in Kerr U.S. Pat. No. 3,671,726, and Dawson U.S. Pat. No. 3,779,647, has also become known, but because of the complexity of laser apparatus, such equipment is undesirably expensive.
Optical measuring devices used in applications other than saw mills and the like where the products whose thickness is to be monitored move continually past a monitoring station depend on counting optical interference fringes produced as light passes through optical gratings. For example, the measuring devices disclosed in Pryor et al U.S. Pat. No. 3,785,737, Brake U.S. Pat. No. 4,079,252, Erickson U.S. Pat. No. 3,768,911, Takeda U.S. Pat. No. 3,833,807, Hayamizu U.S. Pat. No. 3,628,870, Wu U.S. Pat. No. 4,097,150 and deLang U.S. Pat. No. 3,175,093, all employ optical gratings to generate Moire interference fringes which may be electronically counted to determine the magnitude of displacement of one of the gratings. These latter systems, however, are not practical for measuring the thickness of boards and similar type production items because they require the use of optical gratings attached to the surface whose location is to be determined.
Stereoscopic and coincidence optical rangefinders of various types have long been known, but these depend on human operators to measure angular parallax between separate lines of sight and are too inaccurate and slow for use in repeated measurements of materials on a conveyor line.
What is needed, then, is a method for optically measuring the thickness of moving work pieces without physical contact with the work piece or use of coherent light, and using simplified optics for rapid automatic determination of the location of a target surface.